1. Technical Field
This invention generally relates to image transfer technology, and more specifically to electrophotography. The invention is a laser printer developer for liquid toners which minimizes the carry-out of toner liquid.
2. Background Art
In electrophotography, a latent image is created on the surface of an insulating, photo-conducting material by selectively exposing areas of the surface to light. A difference in electrostatic charge density is created between the areas on the surface exposed and unexposed to light. The visible image is developed by electrostatic toners containing pigment components dispersed in an insulating binder. Two types of developer materials are typically employed in the electrostatographic imaging process. The first type of developer material is known as a dry developer material and comprises toner particles, or carrier granules having toner particles adhering tribo-electrically to the carrier granule. The second type of developer material is in the form of a liquid developer, comprising a liquid carrier having toner particles dispersed within the liquid carrier. The toners are selectively attracted to the photoconductor surface areas either exposed or unexposed to light, depending on the relative electrostatic charges of the photoconductor surface, development electrode and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles. For laser printers, the preferred embodiment is that the photoconductor and toner have the same type, but different levels of charge.
Another type of electro statographic printing is known as ion injection or ion deposition printing. In this printing process, the electrostatic latent image is formed on an insulator which receives the ion from the print head.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and passed close to the photoconductor surface, pulling the toner from the photoconductor surface onto the paper or intermediate medium still in the pattern of the image developed from the photoconductor surface. Thermal energy may also be used to assist transfer of the image to paper or intermediate transfer medium. For the case where no thermal transfer is used, a set of fuser rollers melts and fixes the toner in the paper subsequent to direct transfer or indirect transfer when using an intermediate transfer medium, producing the printed image.
There is a demand in the laser printer industry for multi-colored images. Responding to this demand, designers have turned to liquid toners, with pigment components and thermoplastic components dispersed in a liquid carrier medium, as described previously. Usually the liquid carrier is composed of aliphatic hydrocarbon liquids. With liquid toners, it has been discovered, the basic printing colors--yellow, magenta, cyan and black, may be applied sequentially to a photoconductor surface, and from there to a sheet of paper or intermediate medium to produce a multi-colored image.
With liquid toners, however, there is a need to remove the liquid carrier medium from the photoconductor surface after the toner has been applied to it. With the liquid carrier medium removed, the photoconductor surface will not transfer the liquid carrier to the paper or to the intermediate medium in the image transfer step(s). Additionally, the removal allows the liquid carrier to be recovered for recycling and reuse in the developer system. Recycling and reuse represent an advantage by providing economy in terms of printing supplies, and eliminating environmental and health concerns from disposal of excess liquid carrier medium.
It is known from U.S. Pat. No. 3,955,533 to employ a reverse direction roller spaced about 50 microns (about 0.002 inches) from the photoconductor surface to shear off the carrier liquid and excess pigmented solids in the region beyond the outer edge of the image to leave relatively clean background areas on the photoconductor surface.
Also, U.S. Pat. No. 3,957,016, discloses a negative toner system using a positive biased reverse roller maintained at a voltage intermediate to the image and background voltages to help clean the background and compact the image on the photoconductor surface.
Additionally, U.S. Pat. No. 4,286,039, teaches a positive toner system using a reverse roller followed by a negatively biased squeegee roller. The squeegee roller serves two functions, it both compacts the latent image and removes excess carrier liquid.
U.S. Pat. Nos. 4,974,027 and 4,999,677 disclose a positively biased reverse roller followed by a negatively biased rigidizing roller followed by a squeegee roller, separate from the rigidizing roller, for removing excess carrier liquid from the image after rigidization. The charge on these rollers may be reversed if the charge on the toner is reversed. In these two patents, an intermediate transfer drum is downstream of the rigidizing roller for receiving the toner image from the photoconductor surface and transferring the image to a sheet of paper.
U.S. Pat. Nos. 4,299,902, Soma et al., 4,985,733 Kurotori et al., 4,392,742 Landa, and 5,352,558 Smith et al. all disclose similar procedures of removal of excess liquid on the photoconductor surface using a squeezable absorption material. Excess liquid can be removed from the photoconductor surface by incorporating microporosity and compliant properties into the materials on a surface of a flexible belt or roller. By using materials with these properties it is possible to achieve higher solid content of toner of an undeveloped image with liquid developer. This solid content may reach the level of 50%-70%, which exhibits some degree of the dryness in the developed image, compared to the image just developed right at the developer station which is roughly 15% using the process of U.S. Pat. No. 5,352,558, where the solid content of the toner in the reservoir is about 2%-3%. While these other processes allow some improvement in decreasing excess liquid carrier, they also possess the disadvantage of increasing the complexity of the imaging process by using liquid absorbing materials which require extra efforts for cleaning and storage of the excess liquid that is collected.
There is still a need in the electrophotography industry for a liquid toner developer which provides a simple and efficient process for producing a rigid image leaving the developer unit which is drier than the available processes (with a solid content greater than 80 wt. %) and suitable for direct contact with the paper or intermediate transfer medium onto which the image will be transferred with minimal release of the liquid carrier vapor into the environment.